In the era of traditional gasoline-powered vehicles, the engine was called the heart of the car because it was the core component for energy conversion. However, in the era of electric vehicles, people are more concerned about battery energy storage and have neglected the importance of the electric motor.
Although electric motors are used in all aspects of life, drive motors have higher requirements in terms of power, torque, heat dissipation, noise, and output pulse, so there is still much to be done in terms of technology. Today, let's take a brief look at some basic knowledge about power motors.
Motors can generally be divided into two categories based on their energy conversion methods: electric motors and generators. Based on the power supply type, several types of motors can be selected, including DC, AC, permanent magnet brushless, and switched reluctance motors.
The main types of motors suitable for driving new energy vehicles fall into three categories: permanent magnet synchronous motors, AC asynchronous motors, and switched reluctance motors. Due to their different structures and control characteristics, they are used in different areas of the automotive market.
AC asynchronous motor structure diagram
In the passenger vehicle sector, the main types of motors currently used are induction (asynchronous) motors and permanent magnet (synchronous) motors. Tesla is a major representative of the former, while the latter is more mainstream, used by BMW and most domestic electric vehicle manufacturers. What are the differences between these two?
A permanent magnet motor is a synchronous motor. Its rotor uses permanent magnets, and the stator generates electromagnetic torque to drive the rotor's magnetic field to rotate around its axis. The magnetic fields of the stator and rotor are synchronized. An induction motor, on the other hand, is an asynchronous motor. It is an AC motor that generates electromagnetic torque by the interaction between the rotating magnetic field formed by the stator windings and the magnetic field of the induced current in the rotor windings, driving the rotor to rotate.
In the commercial vehicle sector, switched reluctance motors are also widely used, and their development prospects are promising. Their main advantage is that they do not require permanent magnet materials, are not dependent on rare earth elements, and do not even use the copper coils commonly found in motors, thus keeping costs relatively low.
Switched reluctance motors have two basic characteristics:
1. Switching capability: The motor must operate in a continuous switching mode.
2. Reluctance: It is a true reluctance motor, with variable reluctance magnetic circuits in both the stator and rotor. More precisely, it is a doubly salient pole motor.
In terms of performance, switched reluctance motors offer highly flexible speed regulation. This can be achieved not only by changing the voltage but also by altering the on and off angles, resulting in a wide speed range and capability. Their torque density and efficiency also approach those of permanent magnet synchronous motors, surpassing those of induction motors.
However, the main drawback of switched reluctance motors is that the torque ripple is still relatively large, and the vibration and noise are also greater than those of other motors. For passenger cars, this problem is obviously unacceptable, so they have not yet been widely used.
Let's take a closer look at the characteristics of AC asynchronous motors.
The development history of induction (asynchronous) motors:
Nikola Tesla, a pioneer of electricity, invented three-phase alternating current (power electricity) to compete with Edison's direct current system. He also invented a three-phase AC asynchronous motor, which is the ancestor of the power heart of Tesla cars.
In addition, his inventions include the Tesla coil, the Tesla effect, the Tesla transformer (alternating voltage long-range power transmission technology), the Tesla wireless remote control system, and the famous Niagara Falls hydroelectric power station, all of which originated from his research. He earned approximately 1,000 inventions throughout his life, spanning various fields of science and engineering.
The fact that Tesla cars are named after this person is a tribute to this outstanding contributor, and it also signifies that Tesla will definitely continue to follow this path of electric motors and carry it forward.
The powertrain of an electric vehicle is completely different from that of an internal combustion engine vehicle. It typically consists of an energy storage device (ESS) that stores electrical energy, which outputs energy to a converter and a power control module (PEM). The PEM uses sensors to sense the driver's operational needs and road conditions to drive the electric motor.
Energy Storage (ESS)
Automotive energy storage systems can only store direct current (DC), while induction motors use alternating current (AC). To power them, the DC must first be converted to AC, a function performed by the power electronics module (PEM).
Converter and power control module (PEM)
Tesla vehicles' power electronics module uses 72 insulated-gate bipolar transistors (IGBTs) to convert direct current (DC) to alternating current (AC). In addition to controlling the charging and discharging rates, the power electronics module also controls voltage levels, the motor's RPM (revolutions per minute), torque, and the regenerative braking system. This braking system typically captures kinetic energy through braking and feeds it back to the ESS (energy recovery system).
Power output motor:
The battery pack supplies power to the power electronics module to control the speed of the motor (AC induction (asynchronous) motor) and drive the car.
The structural principle of an induction (asynchronous) motor:
An electric motor generally consists of a stator (the stationary part), a rotor (the rotating part that generates kinetic energy), a frame (the housing that connects the stator and rotor), and heat dissipation components.
Basic structure diagram of electric motor
Tesla uses induction (asynchronous) motors, or more precisely, 3-phase (stator windings are connected in a delta or star configuration) and 4-pole induction motors (the number of pole pairs is an important parameter of a motor; a two-pole motor has one number of pole pairs, and a four-pole motor has two number of pole pairs).
The motor speed n = (1-S)60f/P, where S is the motor slip, f is the power supply frequency, and P is the number of pole pairs. When the number of pole pairs is fixed, controlling the motor speed only requires controlling the frequency of the energizing voltage. Controlling the energizing frequency of the stator coils generates a rotating electromagnetic field of varying strength. This field moves relative to the induced magnetic field of the rotor windings, causing the rotor windings to cut magnetic lines of force and generate an induced electromotive force, thus inducing a current in the rotor windings. The induced current in the rotor windings interacts with the magnetic field to produce electromagnetic torque, causing the rotor to rotate.
This part of the structure is a bit difficult to understand. You can simply think of it as controlling the frequency of the alternating current to change the speed of the electric motor and drive the car.
The stator windings can be connected in a delta or star configuration as shown in the figure:
Four-pole wiring method:
Slip between stator and rotor:
Advantages:
① It can withstand large changes in operating temperature. Conversely, large changes in temperature can damage permanent magnet motors.
❷ The output torque of an induction motor can be adjusted over a wide range, thus eliminating the need for a second or even third transmission mechanism. Tesla's motors can reach speeds of 12,000 rpm and produce a maximum torque of 400 Nm, allowing for a forced increase in output torque during acceleration or hill climbing (although for a very short time). In contrast, electric vehicles using permanent magnet motors rely on gearboxes to output more torque for acceleration.
❸ Small size. Currently, most electric vehicle motors are still water-cooled. However, water-cooled motors are larger because the water channels take up too much space.
Tesla's use of induction motors allows for a size comparable to a watermelon. This design offers the advantage of faster heat dissipation; the impact of motor cooling on motor size should not be overlooked. This design, with its superior heat exchange mechanism, allows for greater miniaturization, eliminating the need for radiators, cooling fans, water pumps, and other heat dissipation mechanisms found in other electric vehicles, thus reducing heat loss.
The small size brings another advantage. As we all know, motor power = speed * torque. By making the motor smaller, we can reduce the size of the motor while maintaining the same power, increase the speed of the motor, and ensure low-speed (starting) torque.
❹ Lightweight. There's a saying in the automotive industry: "Better to lose a pound than gain ten horsepower." Tesla's motor is about the size of a watermelon and weighs only 52 kilograms. Its speed range can reach 0-12,000 rpm, so there's no need for an extra transmission mechanism. Therefore, its vehicle weight has a significant advantage.
shortcoming:
① In the past, the biggest drawback of induction asynchronous motors was the difficulty in controlling the rotor's rotational speed, but with the development of modern semiconductor control technology, this problem has been solved.
❷ Because asynchronous induction motors are unilaterally excited, they require a large current to generate a unit torque, and there is reactive excitation current in the stator, resulting in high energy consumption and a lagging power factor.
③ The disadvantages are its complex structure, the use of an induction (asynchronous) motor, and the complex and technically demanding control system, resulting in high manufacturing costs. Therefore, Tesla cars are expensive and unlikely to become affordable for the general public in the short term.
Small Appliance Review:
As the inventor of alternating current, Tesla had an extraordinary passion and ability to control it, and the Tesla Motor is a representative of AC asynchronous motors. Clearly, Tesla cars will follow in the footsteps of its predecessors.
With the rapid development of modern electronic chips, many problems that were difficult to overcome in the past have been gradually solved. Tesla has achieved a high level of speed control for AC asynchronous motors, but there is still room for improvement in reducing energy consumption and increasing power.
